The present invention relates generally to high temperature resin systems and more particularly relates to PMR-type polyimide resin systems exhibiting lower melt viscosity than conventional PMR-type polyimides.
High performance polymer matrix composites (PMC) typically possess high thermal-oxidative stabilities (TOS) and high glass transition temperatures (Tg) to withstand use temperatures up to 316xc2x0 C. (600xc2x0 F.). The use of PMC""s could lead to substantial weight savings over metals, which is an attractive feature for aircraft engines, cryogenic fuel tanks, and many other aerospace applications. This weight reduction translates into better fuel economy, increased speed, and increased passenger load.
In 1972, an improved process, known as in-situ Polymerization of Monomer Reactants (PMR) for polyimide composite fabrication was developed by NASA. The PMR process essentially comprises dissolving a monoalkyl ester of 5-norbornene-2,3-dicarboxylic acid, also known as nadic ester (NE), an aromatic diamine, and a dialkyl ester of an aromatic tetracarboxylic acid in a low-boiling alkyl alcohol such as methanol or ethanol. The monomeric solution can be used to impregnate other components such as reinforcing fibers, with in-situ polymerization through the nadic ester end group occurring directly on the fiber surfaces, producing a composite material with excellent thermal and mechanical properties.
Unfortunately, typical PMR-type polyimides are linear addition-cured polyimides that exhibit high melt flow viscosities well above 100,000 centipoise (cP). The melt flow viscosity limits their processing to techniques involving hand lay-up followed by autoclave or compression molding. These other processing techniques are extremely labor intensive and result in high manufacturing costs for components made with PMR-type polyimides.
Resin Transfer Molding (RTM) provides an economical alternative to hand lay-up based processing, but requires a melt viscosity less than about 4000 cP. This is much lower than the minimum melt viscosity of, for example, PMR-15, which is about 250,000 cP. Thus, there exists a need in the art to provide PMR-type polyimides exhibiting increased processability due, at least in part, to a lowered melt viscosity.
It has been found that PMR-type polyimides can be provided exhibiting lower melt viscosities than PMR-type polyimides of the prior art. These PMR-type polyimides are created by incorporating flexible linkages, such as kinked structures and twisted or non-coplanar moietes into the backbone structure of the PMR. Specifically, the focus of the present invention concerns the production of PMR-type polyimides having 2,2xe2x80x2-disubstituted biaryls in the polymer backbone.
The 2,2xe2x80x2-disubstituted biaryls in the polymer backbone are provided by utilizing 2,2xe2x80x2-disubstituted biaryl diamine monomers and/or 2,2xe2x80x2-disubstituted biaryl dianhydride monomers to form the PMR-type polyimides in a conventional manner. In one embodiment of the present invention, only the diamine used to form the PMR-type polyimide is a 2,2xe2x80x2-disubstituted biaryl monomer, while the dianhydride may be an aromatic dianhydride as typically employed in creating PMR-type polyimides. In another embodiment of the present invention, only the dianhydride used to form the PMR-type polyimide is a 2,2xe2x80x2-disubstituted biaryl monomer, while the diamine may be any aromatic diamine as typically used to produce PMR-type polyimides. In yet another embodiment of the present invention, both the diamines and the dianhydrides employed to produce PMR-type polyimides according to the present invention are 2,2xe2x80x2-disubstituted biaryl monomers. The PMR-type polyimides of the present invention are produced through conventional methods, such as solution polymerization and melt polymerization.
In one embodiment of the present invention is provided an addition-cured polyimide comprising the reaction product of an aromatic diamine; a reactive end group selected from the group consisting of 5-norbornene-2,3-dicarboxylic acid, ester derivatives of 5-norbornene-2,3-dicarboxylic acid, anhydride derivatives of 5-norbornene-2,3-dicarboxylic acid, and 4-phenylethynylphthalic anhydride; and a 2,2xe2x80x2-biaryl dianhydride selected from the group consisting of: 
where each X is the same or different and is selected from O, CH2, CO, SO2, CF2, C(CH3)2, C(CF3)2, C(Ph)(CF3), or nil. It will be appreciated that xe2x80x9cPhxe2x80x9d represents a phenyl group, i.e. C6H5, as is generally known in the art.
In another embodiment, an addition-cured polyimide is provided comprising the reaction product of an aromatic dianhydride; a reactive end group selected from the group consisting of 5-norbornene-2,3-dicarboxylic acid, ester derivatives of 5-norbornene-2,3-dicarboxylic acid, anhydride derivatives of 5-norbornene-2,3-dicarboxylic acid, and 4-phenylethynylphthalic anhydride; and a 2,2xe2x80x2-biaryl diamine selected from the group consisting of: 
where each X is the same or different and is selected from O, CH2, CO, SO2, CF2, C(CH3)2, C(CF3)2, C(Ph)(CF3), or nil.
In yet another embodiment of the present invention is provided in addition-cured polyimide comprising the reaction product of a 2,2xe2x80x2-biaryl anhydride selected from the group consisting of: 
and
a 2,2xe2x80x2-biaryl diamine selected from the group consisting of: 
where each X is the same or different and is selected from O, CH2, CO, SO2, CF2, C(CH3)2, C(CF3)2, C(Ph)(CF3), or nil; and a reactive end group selected from the group consisting of 5-norbornene-2,3-dicarboxylic acid, ester derivatives of 5-norbornene-2,3-dicarboxylic acid, anhydride derivatives of 5-norbornene-2,3-dicarboxylic acid, and 4-phenylethynylphthalic anhydride.
In still a further embodiment of the present invention is provided an addition-cured polyimide according to the following structure: 
wherein n is from 1 to 10, and Z is selected from the group consisting of aromatic dianhydride radicals, and the following structures: 
and Y is selected from the group consisting of aromatic diamine radicals, and the following structures: 
where each X is the same or different and is selected from O, CH2, CO, SO2, CF2, C(CH3)2, C(CF3)2, C(Ph)(CF3), or nil, with the proviso that at least one of Z or Y is selected from the given 2,2xe2x80x2-biaryl structures.